I pressed something, and my object went curved
You most likely added a sub-surface modifier on your object. You can do this by pressing 

Ctrl+1 throughCtrl+5, adding a sub-division surface with the respective amount of divisions.

Because you were pressing Alt+2 before it happened, your finger probably landed on Ctrl when it slipped.

In your second screenshot, you do not have your object selected, so the modifier would not display. in the modifiers panel

You can remove it by going to the modifiers panel and deleting the modifier.